The spirochete Borrelia burgdorferi is the causative agent of Lyme disease. Infected mammals produce bacteriocidal antibodies against B. burgdorferi surface proteins, and passive transfer of sera from infected humans and laboratory animals can protect nave animals against B. burgdorferi challenge. Yet antibodies directed against B. burgdorferi often cannot effectively clear infection, and bacteria can persistently infect humans and other mammals with periodic recurrence of symptoms. These observations suggest that B. burgdorferi alters its surface properties during mammalian infection to evade clearance by the host immune system. Consistent with this prediction, it is well known that B. burgdorferi can regulate the synthesis of antigenic surface proteins, both in vivo and in vitro. Within the first four weeks of infection, mammals produce antibodies against the B. burgdorferi Erp lipoproteins, indicating that the bacteria synthesize Erp proteins during this time period. It is proposed that Erp proteins are involved in the initial infection process by enabling B. burgdorferi to be transmitted from the tick vector and/or interact with mammalian tissues. Infected mammals consistently produce antibodies directed against Erp proteins, but can still be chronically infected by B. burgdorferi. As a corollary to the central hypothesis, it is predicted that Erp proteins are not produced during the later stages of mammalian infection. B. burgdorferi can regulate the synthesis of Erp proteins in culture, and it is hypothesized that Erp protein production is also regulated by similar mechanisms in vivo. The specific aims of the proposed studies are: (1) Characterize erp genes and their proteins in cultured bacteria by investigating the in vitro differential expression of erp mRNAs and proteins, the ability of individual bacteria to simultaneously express their entire repertoire of Erp proteins, the surface accessibility of each Erp protein, and the ability of antibodies directed against Erp proteins to inhibit B. burgdorferi growth. (2) Analyze erp genes and their proteins during mammal and tick infections by examining levels of erp transcripts and Erp proteins throughout the mammal-tick infectious cycle. (3) At this time, essentially nothing is known about the regulatory mechanisms that govern expression of the erp genes or any other B. burgdorferi gene. Cultured bacteria will be used to further characterize mechanisms of erp gene regulation, including the continued purification of a B. burgdorferi protein that specifically binds to erp promoter DNA.